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Astron. Astrophys. 322, 835-840 (1997)

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Wavelet analysis of stellar differential rotation

I. The Sun

A. Hempelmann 1 and R.A. Donahue 2, 3

1 Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany
2 Mount Wilson Institute, 740 Holladay Road, Pasadena, CA 91106, USA
3 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 15, Cambridge, MA 02138, USA

Received 6 September 1996 / Accepted 19 December 1996

Abstract

A key mystery in stellar magnetohydrodynamics is knowledge of stellar differential rotation. At present, the only practical method for detecting the presence of surface differential rotation for slowly-rotating stars is through the indirect measurement of the stellar butterfly diagram: i.e., the change of rotation period determined from light modulation caused by the change in mean latitude of stellar surface inhomogeneities, correlated with the phase of the stellar activity cycle. However, finding the correct period is hampered by stellar active region evolution which introduces variability in the amplitude and phase of the modulation. We demonstrate that time-frequency analysis using the wavelet transform is possibly a suitable method for monitoring the stellar butterfly diagram. Wavelet analysis of disk-integrated solar Ca II K line core emission measures shows nearly the correct pattern of the solar surface differential rotation when viewed as a star. This technique can therefore be applied to long-term measurements of stellar activity.

Key words: Sun: rotation – stars: late-type – stars: rotation – methods: data analysis

Send offprint requests to: Alexander Hempelmann (ahempelmann@aip.de)

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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